1. Field of the Invention
The present invention relates to a semiconductor device and its method of manufacture, and in particular, relates to a semiconductor device in which there is contact between a polysilicon electrode and metal electrode in a gate region, and a manufacturing method for such a semiconductor device.
2. Description of the Related Art
FIG. 1 is a plan view of a MOS transistor of the prior art. The prior-art MOS transistor is formed from gate electrode 97 composed of polysilicon, drain region 94, and source region 95. Gate electrode 97, drain region 94 and source region 95 are each connected to metal wiring 91 by means of contacts 99. Contacts 99 are formed on element isolation region 93 formed from a thick silicon oxide film that covers the field region. However, gate electrode 97 of the transistor is fabricated at close to the limits of accuracy of methods such as lithographic processing, and the formation of contacts 99 in the gate region is therefore difficult. Contacts 99 are therefore provided on the element isolation region 93 apart from the gate region.
However, in a semiconductor device in which an electrode formed from polysilicon is provided over a wide area, such as in a CCD (Charge Coupled Device) solid-state image-sensing device, a polysilicon electrode has a sheet resistance on the order of 20.about.50 .OMEGA./.quadrature.. This sheet resistance can be reduced by providing a metal electrode on the rear surface of the polysilicon electrode and by providing contacts for connection with the metal electrode directly over the polysilicon electrode.
FIG. 2 is a schematic sectional view of this type of prior-art solid-state image-sensing device. In this prior-art solid-state image-sensing device, charge transfer electrodes 108 and 110 are provided on diffusion region 100. Change to lower sheet resistance is then brought about by first connecting polysilicon wiring 101 with charge transfer electrode 108 or 110 by way of contact 101a, and connecting aluminum layer 103 with this polysilicon wiring 101 by way of contact 103a.
Here, a method of connecting aluminum layer 103 with charge transfer electrode 108 or 110 by means of contact 103a has also been considered, but such a method could not be employed because aluminum layer 103 tends to diffuse within polysilicon electrode and therefore deposits on the gate insulation film surface partially, thereby resulting in such problems as alteration of the silicon surface potential and lowering of the dielectric strength of the oxide film.
FIG. 3 is a schematic sectional view showing a prior-art solid-state image-sensing device for solving the above-described problems.
This solid-state image-sensing device of the prior art is described at pages 105.about.108 of the preliminary papers of the 1992 IEDM (International Electronic Devices Meeting).
This solid-state image-sensing device of the prior art employs tungsten layer 105 in a shield layer, this tungsten layer 105 being connected with charge transfer electrode 108 and charge transfer electrode 110 by way of contacts 105a. Tungsten layer 105 simultaneously serves as a shield layer and wiring.
Tungsten is not as prone as aluminum to diffuse into a polysilicon electrode, and as a result, when tungsten layer 105 is used as wiring, it can directly connect with charge transfer electrodes 108 and 110 by way of contact 105a.
This solid-state image-sensing device of the prior art enables a greater reduction of the sheet resistance than the solid-state image-sensing device of FIG. 2 by using tungsten layer 105 for wiring in place of an aluminum layer. This element also provides the effect of greatly reducing the series parasitic resistance of the power source of charge transfer electrodes 108 and 110. The preliminary papers of the IEDM mentioned hereinabove also include a description of a CCD image sensor that applies the solid-state image-sensing device of FIG. 3.
In the above-described semiconductor devices of the prior art, however, a conductive film of, for example, tungsten comes in direct contact with a silicon electrode composed of polysilicon or single-crystal silicon, and CCD solid-state image-sensing devices of this construction therefore entail problems such as the occurrence of changes in threshold value or channel potential as well as drops in the charge transfer efficiency.